Incoherent Mollow triplet

A counterpart of the Mollow triplet (luminescence lineshape of a two-level system under coherent excitation) is obtained for the case of incoherent excitation in a cavity. Its analytical expression, in excellent agreement with numerical results, pinpoints analogies and differences between the conventional resonance fluorescence spectrum and its cavity QED analogue under incoherent excitation.Comment: 4 pages, 3 figure

On the Evolution of the Cosmic Supernova Rates

Ongoing searches for supernovae (SNe) at cosmological distances have recently started to provide a link between SN Ia statistics and galaxy evolution. We use recent estimates of the global history of star formation to compute the theoretical Type Ia and Type II SN rates as a function of cosmic time from the present epoch to high redshifts. We show that accurate measurements of the frequency of SN events in the range 0<z<1 will be valuable probes of the nature of Type Ia progenitors and the evolution of the stellar birthrate in the universe. The Next Generation Space Telescope should detect of order 20 Type II SNe per 4'x 4' field per year in the interval 1<z<4.Comment: LaTeX, 19 pages, 3 figures, to be published in the MNRA

On Core Collapse Supernovae in Normal and in Seyfert Galaxies

This paper estimates the relative frequency of different types of core-collapse supernovae, in terms of the ratio f between the number of type Ib--Ic and of type II supernovae. We estimate f independently for all normal and Seyfert galaxies whose radial velocity is <=14000 km/s, and which had at least one supernova event recorded in the Asiago catalogue from January 1986 to August 2000. We find that the ratio f is approx. 0.23+/-0.05 in normal galaxies. This value is consistent with constant star formation rate and with a Salpeter Initial Mass Function and average binary rate approx. 50 %. On the contrary, Seyfert galaxies exceed the ratio f in normal galaxies by a factor approx. 4 at a confidence level >= 2 sigma. A caveat is that the numbers for Seyferts are still small (6 type Ib-Ic and 6 type II supernovae discovered as yet). Assumed real, this excess of type Ib and Ic with respect to type II supernovae, may indicate a burst of star formation of young age (<= 20 Myr), a high incidence of binary systems in the inner regions (r <= 0.4 R25) of Seyfert galaxies, or a top-loaded mass function.Comment: Accepted for Publication in MNRA

Cumulative physical uncertainty in modern stellar models. II. The dependence on the chemical composition

We extend our work on the effects of the uncertainties on the main input physics for the evolution of low-mass stars. We analyse the dependence of the cumulative physical uncertainty affecting stellar tracks on the chemical composition. We calculated more than 6000 stellar tracks and isochrones, with metallicity ranging from Z = 0.0001 to 0.02, by changing the following physical inputs within their current range of uncertainty: 1H(p,nu e+)2H, 14N(p,gamma)15O and triple-alpha reaction rates, radiative and conductive opacities, neutrino energy losses, and microscopic diffusion velocities. The analysis was performed using a latin hypercube sampling design. We examine in a statistical way the dependence on the variation of the physical inputs of the turn-off (TO) luminosity, the central hydrogen exhaustion time (t_H), the luminosity and the helium core mass at the red-giant branch (RGB) tip, and the zero age horizontal branch (ZAHB) luminosity in the RR Lyrae region. For the stellar tracks, an increase from Z = 0.0001 to Z = 0.02 produces a cumulative physical uncertainty in TO luminosity from 0.028 dex to 0.017 dex, while the global uncertainty on t_H increases from 0.42 Gyr to 1.08 Gyr. For the RGB tip, the cumulative uncertainty on the luminosity is almost constant at 0.03 dex, whereas the one the helium core mass decreases from 0.0055 M_sun to 0.0035 M_sun. The dependence of the ZAHB luminosity error is not monotonic with Z, and it varies from a minimum of 0.036 dex at Z = 0.0005 to a maximum of 0.047 dex at Z = 0.0001. Regarding stellar isochrones of 12 Gyr, the cumulative physical uncertainty on the predicted TO luminosity and mass increases respectively from 0.012 dex to 0.014 dex and from 0.0136 M_sun to 0.0186 M_sun. Consequently, for ages typical of galactic globular clusters, the uncertainty on the age inferred from the TO luminosity increases from 325 Myr to 415 Myr.Comment: Accepted for publication in A&

On the age of Galactic bulge microlensed dwarf and subgiant stars

Recent results by Bensby and collaborators on the ages of microlensed stars in the Galactic bulge have challenged the picture of an exclusively old stellar population. However, these age estimates have not been independently confirmed. In this paper we verify these results by means of a grid-based method and quantify the systematic biases that might be induced by some assumptions adopted to compute stellar models. We explore the impact of increasing the initial helium abundance, neglecting the element microscopic diffusion, and changing the mixing-length calibration in theoretical stellar track computations. We adopt the SCEPtER pipeline with a novel stellar model grid for metallicities [Fe/H] from -2.00 to 0.55 dex, and masses in the range [0.60; 1.60] Msun from the ZAMS to the helium flash at the red giant branch tip. We show for the considered evolutionary phases that our technique provides unbiased age estimates. Our age results are in good agreement with Bensby and collaborators findings and show 16 stars younger than 5 Gyr and 28 younger than 9 Gyr over a sample of 58. The effect of a helium enhancement as large as Delta Y/Delta Z = 5 is quite modest, resulting in a mean age increase of metal rich stars of 0.6 Gyr. Even simultaneously adopting a high helium content and the upper values of age estimates, there is evidence of 4 stars younger than 5 Gyr and 15 younger than 9 Gyr. For stars younger than 5 Gyr, the use of stellar models computed by neglecting microscopic diffusion or by assuming a super-solar mixing-length value leads to a mean increase in the age estimates of about 0.4 Gyr and 0.5 Gyr respectively. Even considering the upper values for the age estimates, there are four stars estimated younger than 5 Gyr is in both cases. Thus, the assessment of a sizeable fraction of young stars among the microlensed sample in the Galactic bulge appears robust.Comment: Accepted for publication in A&A. Abstract shortene

Mixing-length estimates from binary systems. A theoretical investigation on the estimation errors

We performed a theoretical investigation on the mixing-length parameter recovery from an eclipsing double-lined binary system. We focused on a syntetic system composed by a primary of mass M = 0.95 Msun and a secondary of M = 0.85 Msun. Monte Carlo simulations were conducted at three metallicities, and three evolutionary stages of the primary. For each configuration artificial data were sampled assuming an increasing difference between the mixing-length of the two stars. The mixing length values were reconstructed using three alternative set-ups. A first method, which assumes full independence between the two stars, showed a great difficulty to constrain the mixing-length values: the recovered values were nearly unconstrained with a standard deviation of 0.40. The second technique imposes the constraint of common age and initial chemical composition for the two stars in the fit. We found that $\alpha_{ml,1}$ values match the ones recovered under the previous configuration, but $\alpha_{ml,2}$ values are peaked around unbiased estimates. This occurs because the primary star provides a much more tight age constraint in the joint fit than the secondary. Within this second scenario we also explored, for systems sharing a common $\alpha_{ml}$, the difference in the mixing-length values of the two stars only due to random fluctuations owing to the observational errors. The posterior distribution of these differences was peaked around zero, with a large standard deviation of 0.3 (15\% of the solar-scaled value). The third technique also imposes the constraint of a common mixing-length value for the two stars, and served as a test for identification of wrong fitting assumptions. In this case the common mixing-length is mainly dictated by the value of $\alpha_{ml,2}$. [...] For $\Delta \alpha_{ml} > 0.4$ less than half of the systems can be recovered and only 20% at $\Delta \alpha_{ml} = 1.0$.Comment: Abstract abridge

Evolution of the habitable zone of low-mass stars. Detailed stellar models and analytical relationships for different masses and chemical compositions

We study the temporal evolution of the habitable zone (HZ) of low-mass stars - only due to stellar evolution - and evaluate the related uncertainties. These uncertainties are then compared with those due to the adoption of different climate models. We computed stellar evolutionary tracks from the pre-main sequence phase to the helium flash at the red-giant branch tip for stars with masses in the range [0.70 - 1.10] Msun, metallicity Z in the range [0.005 - 0.04], and various initial helium contents. We evaluated several characteristics of the HZ, such as the distance from the host star at which the habitability is longest, the duration of this habitability, the width of the zone for which the habitability lasts one half of the maximum, and the boundaries of the continuously habitable zone (CHZ) for which the habitability lasts at least 4 Gyr. We developed analytical models, accurate to the percent level or lower, which allowed to obtain these characteristics in dependence on the mass and the chemical composition of the host star. The metallicity of the host star plays a relevant role in determining the HZ. The initial helium content accounts for a variation of the CHZ boundaries as large as 30% and 10% in the inner and outer border. The computed analytical models allow the first systematic study of the variability of the CHZ boundaries that is caused by the uncertainty in the estimated values of mass and metallicity of the host star. An uncertainty range of about 30% in the inner boundary and 15% in the outer one were found. We also verified that these uncertainties are larger than that due to relying on recently revised climatic models, which leads to a CHZ boundaries shift within 5% with respect to those of our reference scenario. We made an on-line tool available that provides both HZ characteristics and interpolated stellar tracks.Comment: Accepted for publication in A&A, abstract abridge

Uncertainties on the theoretical predictions for classical Cepheid pulsational quantities

The expected distribution of Cepheids within the instability strip is affected by several model inputs, reflecting upon the predicted Period-Luminosity relation. On the basis of new and updated sets of evolutionary and pulsational models, we quantitatively evaluated the effects on the theoretical PL relation of current uncertainties on the chemical abundances of Cepheids in the Large Magellanic Cloud and on several physical assumptions adopted in the evolutionary models. We analysed how the different factors influence the evolutionary and pulsational observables and the resulting PL relation. As a result, we found that present uncertainties on the most relevant H and He burning reaction rates do not influence in a relevant way the loop extension in temperature. On the contrary, current uncertainties on the LMC chemical composition significantly affect the loop extension and also reflect in the morphology of the instability strip; however their influence on the predicted pulsational parameters is negligible. We also discussed how overshooting and mass loss influence the ML relation and the pulsational parameters. In summary, the present uncertainties on the physical inputs adopted in the evolutionary codes and in the LMC chemical composition are negligible for the prediction of the main pulsational properties; the inclusion of overshooting in the previous H burning phase and/or of mass loss is expected to significantly change the resulting theoretical pulsational scenario for Cepheids, as well as the calibration of their distance scale. These systematic effects are expected to influence the theoretical Cepheid calibration of the secondary distance indicators and in turn the resulting evaluation of the Hubble constant.Comment: accepted for publication on A&

Theoretical investigation on the mass loss impact on asteroseismic grid-based estimates of mass, radius, and age for RGB stars

We aim to perform a theoretical evaluation of the impact of the mass loss indetermination on asteroseismic grid based estimates of masses, radii, and ages of stars in the red giant branch phase (RGB). We adopted the SCEPtER pipeline on a grid spanning the mass range [0.8; 1.8] Msun. As observational constraints, we adopted the star effective temperatures, the metallicity [Fe/H], the average large frequency spacing $\Delta \nu,$ and the frequency of maximum oscillation power $\nu_{\rm max}$. The mass loss was modelled following a Reimers parametrization with the two different efficiencies $\eta = 0.4$ and $\eta = 0.8$. In the RGB phase, the average error owing only to observational uncertainty on mass and age estimates is about 8% and 30% respectively. The bias in mass and age estimates caused by the adoption of a wrong mass loss parameter in the recovery is minor for the vast majority of the RGB evolution. The biases get larger only after the RGB bump. In the last 2.5% of the RGB lifetime the error on the mass determination reaches 6.5% becoming larger than the random error component in this evolutionary phase. The error on the age estimate amounts to 9%, that is, equal to the random error uncertainty. These results are independent of the stellar metallicity [Fe/H] in the explored range. Asteroseismic-based estimates of stellar mass, radius, and age in the RGB phase can be considered mass loss independent within the range ($\eta \in [0.0, 0.8]$) as long as the target is in an evolutionary phase preceding the RGB bump.Comment: Accepted for publication in A&

A Radial Velocity Survey for LMC Microlensed Sources

We propose a radial velocity survey with the aim to resolve the current dispute on the LMC lensing: in the pro-macho hypothesis the lenses are halo white dwarfs or machos in general; in the pro-star hypothesis both the lenses and the sources are stars in various observed or hypothesized structures of the Magellanic Clouds and the Galaxy. Star-star lensing should prefer sources at the backside or behind the LMC disc because lensing is most efficient if the source is located a few kpc behind a dense screen of stars, here the thin disc of the LMC. This signature of self-lensing can be looked for by a radial velocity survey since kinematics of the stars at the back can be markedly different from that of the majority of stars in the cold, rapidly rotating disc of the LMC. Detailed simulations of effect together with optimal strategies of carrying out the proposed survey are reported here. Assuming that the existing 30 or so alerted stars in the LMC are truely microlensed stars, their kinematics can test the two lensing scenarios; the confidence level varies with the still very uncertain structure of the LMC. Spectroscopy of the existing sample and future events requires about two or three good-seeing nights per year at a 4m-8m class southern telescope, either during the amplification phase or long after.Comment: minor changes of text, ApJ accepte